How can I implement a min-heap of f64 with Rust's BinaryHeap?

Question

I want to populate a binary heap with floats--more specifically, I\'d like to implement a min-heap.

It seems that floats do not support Ord and thus are

Answer 1

Instead of writing your own MinNonNan, consider using the ordered-float crate + the std::cmp::Reverse type.

type MinNonNan = Reverse<NotNan<f64>>;

---

Since you are #[derive]ing PartialOrd, the .gt(), .lt() etc methods still compare normally, i.e. MinNonNan(42.0) < MinNonNan(47.0) is still true. The Ord bound only restricts you to provide strictly-ordered types, it doesn't mean the implementation will use .cmp() instead of </>/<=/>= everywhere, nor the compiler will suddenly change those operators to use the Ord implementation.

If you want to flip the order, you need to reimplement PartialOrd as well.

#[derive(PartialEq)]
struct MinNonNan(f64);

impl PartialOrd for MinNonNan {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        other.0.partial_cmp(&self.0)
    }
}

impl Ord for MinNonNan {
    fn cmp(&self, other: &MinNonNan) -> Ordering {
        self.partial_cmp(other).unwrap()
    }
}

Answer 2

Working Example

use std::cmp::Ordering;
use std::collections::BinaryHeap;

#[derive(PartialEq)]
struct MinFloat(f64);

impl Eq for MinFloat {}

impl PartialOrd for MinFloat {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        other.0.partial_cmp(&self.0)
    }
}

impl Ord for MinFloat {
    fn cmp(&self, other: &MinFloat) -> Ordering {
        self.partial_cmp(other).unwrap()
    }
}

fn main() {
    let mut minheap = BinaryHeap::new();
    minheap.push(MinFloat(2.0));
    minheap.push(MinFloat(1.0));
    minheap.push(MinFloat(42.0));
    if let Some(MinFloat(root)) = minheap.pop() {
        println!("{:?}", root);
    }
}